[TIMOB-17015] TiAPI: add runtime directives to distinguish between ad-hoc and app-store distribution

With ALOY-680 resolved on compile-time for Alloy 1.4.0, I'd like to see something similar for the Titanium API as well so I don't have to switch out keys and code when building for the stores, with all risks of forgetting something. For a use case: https://github.com/jeffbonnes/installr-updater/issues/1. What it should look like:

var key;
if (Ti.App.distributionType === Ti.App.DIST_STORE) {
  key = 'myKeyOnlyWorkingInStores';
} else if (Ti.App.distributionType === Ti.App.DIST_ADHOC) {
  key = 'myKeyForAdHocBuildsToTesters';
} else {
  key = 'myKeyForLocalTesting';
}